This invention relates to a receiver of the type capable of receiving multiplexed pulse tone signals.
In certain alarm and time-of-event telemeter systems, a short duration signal pulse is transmitted from a remote location to a central receiver. If a number of remote locations are represented by individual tone pulse transmitters having different modulation tones and which are triggered by external means, such as relay or switch contacts, then the position of the transmitter is identified by its unique tone frequency and the time-of-event by the reception of the first pulse signal. The central receiver processes these signals, resulting in an alarm or telemetering system for time-of-event and position signalling.
Because of the random occurrence of the detected events and the plurality of positions required, the signalling pulses should be of short duration compared to their repetition rates, thus allowing a large number of multiplexed time slots.
In this as well as other applications, if tone signals are transmitted as brief, spaced apart burts, that is, if the tone signals are transmitted as pulse tone signals, the power requirements of the transmitter are significantly reduced. However, if the pulse tone signal is relatively narrow, the tone detectors in the central receiver must be capable of responding fast enough so as to detect the presence of corresponding tone signals during the brief, respective intervals that they are present. Furthermore, if the pulse tone signals are transmitted by radiant energy, such as by radio transmitters, rather than via hard-wired systems, the possibility of interference in such radio transmissions requires that the tone detectors exhibit the characteristics of rapidly responding, narrow band filters.
While active filters are known to exhibit desirably narrow pass bands, such active filters generally do not respond with sufficient speed. That is, the inherent time delay of such filters is a disadvantage for use in the aforementioned applications. Digital filter circuits may exhibit favorable response and pass band characteristics; but digital filters generally are relatively expensive to implement.
Furthermore, in applications of the aforementioned type wherein tone signals of different frequencies are to be detected, it is advantageous to provide a filter of basic construction but that is programmable so as to pass different ones of these tone signals as desired. The same unit, or filter, thus can be "programmed" to pass one tone frequency, and as conditions arise or uses change, that same unit can be "re-programmed" to pass a different tone frequency. The aforementioned active filters, unfortunately, are not programmable.